Modern cardiac pacing systems have incorporated substantial capability for detecting and dealing with various arrhythmias. Of particular importance are atrial arrhythmias such as atrial fibrillation (AF), which is extremely dangerous to the patient. Pacemaker have attempted to deal with such arrhythmias by simply switching into an asynchronous mode, such that ventricular pacing does not try to track the dangerous atrial excitations. However, with ordinary asynchronous ventricular pacing and continued conduction of the atrial signals through the A-V node, a certain percentage of the atrial signals will get through to the ventricle and thus cause chaotic spontaneous ventricular contractions and paced contractions, resulting in an undesirable cardiac condition. Thus, patients with occasionally occurring atrial flutter or fibrillation are presently candidates for His bundle ablation, a procedure which aims to effectively alter the cardiac conduction pattern to interrupt the reentry cycles which support the high rate arrhythmia. This is, of course, a procedure which stops conduction of all atrial signals to the ventricle permanently. The result is that the ventricle needs to be paced permanently even though the atrium contracts normally most of the time. Besides the disadvantages of pacing compared to normal physiological contraction, ablation is a potentially dangerous and costly procedure, and it would be vastly preferred to have a less invasive alternative procedure.
Another technique that is in use is that of delivering a cardioversion shock to the patient's heart. This can be done during general anesthesia, which of course is impractical for a patient who has repeat and rather long-occurring episodes. Such a patient would also be a candidate for an implantable cardioverter device. However, such devices are very expensive, and the shocks are not welcome to the patient, i.e., they are painful. Further, if the episodes occur too frequently, these devices have a limited lifetime due to the energy expenditure of each shock.
Another approach known in the literature is to cool the atrium, thereby slowing conduction in the atrial tissue to the point of terminating the atrial fibrillation. See Abstract, Scaglione et al, PACE, Vol. 16, p 880, April 1993, Part II. In this approach, the entire atrium is cooled by introduction of a bolus of cold saline solution. Alternately, it has been proposed to cool the blood of the atrium by use of an electrically driven element. "Implanted Device to Automatically Terminate Atrial Filbrillation By Rapid Cooling of the Right Atrium," Research Disclosure, April 1994/207. However, to date this approach has not proven feasible for an implanted device.
Another prior art approach utilizes cooling of specific locations in the atrium in order to map the location of an arrhythmia source, followed by ablation of the tissue found to be giving rise to the arrhythmia or fibrillation. In U.S. Pat. No. 5,529,067, a catheter with a Peltier element embedded in its distal end is utilized to contact and cool the inside of the atrial wall. By moving the distal tip around and cooling different localized areas, all the time monitoring the atrial activity, the trouble spot which is the source of the arrhythmia can be found and then treated with conventional RF ablation. However, this procedure still requires the ablation therapy, which has the disadvantages set forth above.
There thus remains a substantial need for an improved system and technique for effectively inhibiting such atrial arrhythmias until the atrium can return on its own to a normal sinus rhythm, during which time the ventricle can be paced asynchronously. As used hereinafter, the term atrial fibrillation, or AF, refers broadly to the class of dangerous atrial arrhythmias, during episodes of which it is desired to inhibit conduction to the ventricles and pace asynchronously.